HB-LEDs have gained greater importance in recent years and will become one of the most important products for the lighting industry in the near future. However, because conventional HB-LEDs have a common problem in generating heat flux which is higher than 100 W/cm2, conventional packages designed for indicator LEDs are not suitable for HB-LEDs. Over-heating of an LED would cause premature failure because the efficiency, spectrum, reliability and life of the solid state lighting devices strongly depend on successful thermal management.
Epoxy adhesives are most commonly used in HB-LEDs and in semiconductor packaging because of their good adhesion to different kinds of substrates, ability to automate in ultra-rapid processes, low cost, and ease of use resulting in saving production cycle time, etc.
Unmodified epoxy polymeric resins are natural insulators and exhibit low electrical and thermal conductivity. Thus, appropriate fillers have been used to produce adhesives with high electrical and thermal conductivities. Sufficient electrical/thermal conductive particles should be added to form a network within the polymer matrix such that electrons and heat can flow across the particle contact points in order to make the mixture electrically and thermally conductive. Based on this concept, the thermal conductivity of a conventional die attach adhesive (“DAA”) is mostly achieved by the fillers and the linkage between the fillers through the epoxy resin matrix [M. Inoue, H. Muta, S. Yamanaka, K. Suganuma, Osaka University, Japan, J. Electron. Mater., Vol. 37, No. 4 (2008) pp. 462-468.].
Metal powders, including silver, copper, gold, and aluminum etc., have been used as conventional DAA fillers to achieve a thermal conductivity of about 1-5 W/M·K. However, this thermal conductivity does not meet the current demands of high power devices. The design of filler materials as well as the chemistry between the filler and matrix had been actively studied [Yi Li, C. P. Wong, Materials Science and Engineering R 51, 2006, 1-35; D. D. Lu, Y. G. Li, C. P. Wong, 22, 2008, 801-834]. In addition, further research focuses on novel types of high thermally conductive fillers, including boron nitride, aluminum nitride, and carbon allotropes etc. [C.-W. Nan, G. Liu, Y. Lin, and M. Li, Appl. Phys. Lett., vol. 85, 2004, 3549-3551; C. H. Liu, H. Huang, Y. Wu, and S. S. Fan, Appl. Phys. Lett. 84, 2004, 4248-4250].
Attempts to enhance the thermal conductivity of the epoxy composite have also included the use of nanoparticle materials. For example, U.S. patent application Ser. No. 10/426,485 discloses the use of non-electrically conductive nanoparticles in a polymer matrix to improve the thermal conductivity of a polymer composite system. U.S. Pat. No. 7,550,097 also describes thermal conductive materials utilizing electrically conductive nanoparticles which show less phase separation than the micron-sized particles.
In addition to fillers, CVD grown carbon nanotubes (CNT) to serve as thermal interface materials (TIM) in HP (high-power)-LED (equivalent to HB-LED) packaging to enhance the thermal conductivity of the interface [K. Zhang, M. M. F. Yuen, D. G. W. Xiao, Y. Y. Fu, P. Chan, Proc. 2008 ECTC 58th, pp. 1659-1663; K. Zhang, Y. Chai, M. M. F. Yuen, D. G. W. Xiao, P. C. H. Chan, “Carbon nanotube thermal interface material for high-brightness light-emitting-diode cooling”, Nanotechnology, 19, no. 215706]. However, as compared to the CNT-TIM used on heat-sink surfaces, the die bonding process in LED or other semiconductors still use die attach adhesives for bonding die/chip on different substrates (ceramic, silicon, copper, aluminum, etc). The unstable bonding between CNT-TIM and the die cannot provide enough strength for subsequent wire bonding processes. In contrast, a cured DAA is strong enough for such processes. This kind of cured DAA can link the most pivotal part, the LED die, and the substrate directly. Because of these characteristics, this kind of die attach material not only provides thermal and electrical conductivity between the die and the package but also essentially improves the performance of the device while operating in the field. The functionalized epoxy adhesives dominate the existing market share in mass production because of their good adhesion to different substrates, low cost and ability to automate in the bonding process.
As such, there is a need for developing a new DAA with higher thermal conductivity, better adhesion, and more mechanical stability. Until now, only a few DAAs have been reported which are able to reach a relatively high thermal conductivity of 15-25 W/M·K. DAAs with thermal conductivity of higher than 40 W/M·K with higher stability and relatively low in cost are still needed to form a reliable and stable HB-LED package.
Conventional epoxy adhesives are mostly composed of two initially separated parts: “A” is a linear polymer resin, and “B” is a curing agent. When the “A” part is mixed with “B” part, the linear polymer resin are activated by the curing agent and the linear molecules are cross linked to each other to form a final three-dimensional network. This kind of two-part adhesive has some advantages such as a very long shelf time and very low curing temperature curable at room temperature. However, their disadvantages in electronic devices packaging include an extra mixing process, short working life of the mixed adhesive resulting in a lot of wastes, and difficult application because when the two parts are mixed, they start curing right away at room temperature and the working life is too short which is not suitable for automatic die attach process.
In contrast, one-part epoxy adhesives are known for their versatility in different applications including electronic device packaging. They generally give outstanding adhesion to a wide range of substrates, very high bond strength and have excellent electrical properties. However, one-part epoxy adhesives have some limitations in electronic applications. Firstly, when a polymer resin and a curing agent are mixed together, very low shelf temperatures at ˜−40° C. are needed to lower the reactivity. This makes the reaction mixture more costly and energy-consuming in transportation and storage. Secondly, latent curing agents are mostly used in one-part adhesive. As a result, most of the existing one-part epoxy adhesives need to be cured at a high temperature (≧150° C.) for a long time, which limits their use in LED devices with temperature sensitive parts such as LEDs. Such a cure condition is also a drawback from an energy saving and mass production efficiency point of view. Although some research work has focused on solving this problem by utilizing microencapsulated hardeners [U.S. Pat. No. 7,854,860], this kind of one-part material cannot be used in thermal/electrical conductive adhesives because the hard particle fillers in the adhesives destroy the soft microcapsule shells and the hardeners are being released resulting in the failure of the whole adhesive system. Thus, there remains a need in the art for improved epoxy systems having high thermal and electrical conductivity combined with ease of use, storage convenience, and the ability to form high strength bonds with a wide variety of substrate materials.